1. Field of the Invention
This invention relates to ink jet printing devices, and more particularly to larger silicon thermal ink jet printheads which have ink passageways fabricated by anisotropic etching of silicon. The invention reduces effects of angular misalignment between the etchant resistant mask and the silicon substrate {111} crystal plane in order to provide increased dimensional control of ink passageways and to produce printheads that are more robust without sacrificing resolution.
2. Description of the Prior Art
Thermal ink jet printing is a type of drop-on-demand ink jet system, wherein an ink jet printhead expels ink droplets on demand by the selective application of a current pulse to a thermal energy generator, usually a resistor, located in capillary-filled, parallel ink channels a predetermined distance upstream from the channel nozzles or orifices. The channel ends opposite the nozzles are in communication with an ink reservoir to which an external ink supply is connected.
U.S. Pat. No. Re. 32,572 to Hawkins et al discloses a thermal ink jet printhead and several fabricating processes therefor. Each printhead is composed of two parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes, and the second part is a substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. A linear array of parallel grooves is also formed in the second part, so that one end of each groove communicates with the manifold recess and the other end of each groove is open for use as an ink droplet expelling nozzle. Many printheads can be made simultaneously be producing a plurality of sets of heating element arrays with their addressing electrodes on a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channel grooves and associated manifolds are produced in a second silicon wafer. In one embodiment, alignment openings are etched in the second silicon wafer at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks, then bonded together and diced into many separate printheads.
U.S. Pat. No. 4,638,337 to Torpey et al discloses an improved thermal ink jet printhead similar to that of Hawkins et al, but has each of its heating elements located in a recess. The floor of the recess contains the heating elements, while the recess walls prevent the lateral movement of the bubbles toward the nozzle and, therefore, the sudden release of vaporized ink to the atmosphere, known as blow-out, which causes ingestion of air and interrupts the printhead operation whenever this event occurs. In this patent, a thick film organic structure such as Riston.RTM. or Vacrel.RTM. is interposed between the heater plate and the channel plate. The purpose of this layer is to have recesses formed therein directly above the heating elements to contain the bubble which is formed over the heating elements, thus enabling an increase in the droplet velocity without the occurrence of vapor blow-out and concomitant air ingestion.
U.S. Pat. No. 4,774,530 to Hawkins discloses the use of an etched thick film insulative layer to provide the flow path between the ink channels and the manifold, thereby eliminating the fabrication steps required to open the channel groove closed ends to the manifold recess, so that the printhead fabrication process is simplified.
U.S. Pat. No. 4,786,357 to Campanelli et al, discloses the use of a patterned thick film insulative layer between mated and bonded substrates. One substrate has a plurality of heating element arrays and addresing electrodes formed on the surface thereof and the other being a silicon wafer having a plurality of etched manifolds, with each manifold having a set of ink channels. The patterned thick film layer provides a clearance space above each set of contact pads of the addressing electrodes to enable the removal of the unwanted silicon material by dicing without the need for etched recesses therein. The individual printheads are produced subsequently by dicing the substrate having the heating element arrays.
As disclosed in the above-discussed patents, thermal ink jet printheads are basically fabricated from two substrates. One substrate contains the heating elements and the other contains ink recesses. When these two substrates are aligned and bonded together, the recesses serve as ink passageways. A plurality of each substrate is formed on separate wafers, so that the wafers may be aligned, mated, and diced into many individual printheads. The wafer for the plurality of sets of recesses is silicon and the recesses are formed by an anisotropic etching process. The anisotropic or orientation dependent etching has been shown to be a high yielding fabrication process for precise, miniature printheads. They are low cost, high resolution, electronically addressable printers with high reliability. Such printheads are usually about a quarter of inch wide and print samll swaths of information being translated across a stationary recording medium such as paper. The paper is then stepped the distance of one swath and the printing process continued until the entire page of paper is printed. This is a low speed process.
In efforts to increase the printing speed, larger arrays of nozzles are required. Each ink droplet emitting nozzle requires an ink channel which is in communication with an ink reservoir or manifold. In order to complete the etching from only one side of the wafer, the reservoir is etched through the wafer so that the open bottom may serve as an ink inlet. As the array size increases, so also does the reservoir and thus the ink inlet. As the area of the through etch for the reservoirs increase, the wafer strength diminishes and yield drops because many of the fragile wafers are damaged during subsequent assembly operations.
There is another problem associated with long troughs or recesses. If the sides of the vias formed in the etch resistant masks are not perfectly aligned with the {111} crystal planes of the (100) silicon wafers or substrates, the resulting etched recesses will undercut the mask via and follow the {111} crystal planes nevertheless. Thus, any angular misalignment of the mask relative to the {111} crystal planes of the wafer will result in a rectangular etch recess having longer and wider dimensions than desired, as shown in FIG. 4 discussed later. This undercutting gets more severe as the desired recess or through slot length increases. Since the undercutting is a variable, depending on the pattern-crystal plane misorientation of a particular wafer, it cannot be easily compensated for in the mask design.